Forests in Vermont's Green Mountains transition abruptly from a heat-loving mix of sugar maple, American beech, and yellow birch on the lower slopes to a cold-adapted mix of red spruce, balsam fir, and paper birch higher up.

A new study shows that the altitude of that transition zone rose as much as 400 feet between 1962 and 2005—right in sync with a hike of 2 Fahrenheit degrees in the area's mean annual temperature.

Brian Beckage of the University of Vermont in Burlington and five colleagues documented those changes with aerial photographs, satellite imagery, and on-site measurements. That cold-loving vegetation should retreat up mountain slopes as the climate warms is hardly unexpected. But the researchers were surprised that such a marked shift occurred within just 40 years —less than the natural life span of many trees.

For one forest type to replace another, living trees must die. The resulting vacancy in the canopy allows saplings below to fight it out for supremacy. Although a 2-degree temperature change would undoubtedly influence the results of the sapling competition, it's unlikely by itself to have killed off the mature trees first. For that, Beckage's team suspects the acid rain that's been falling since the 1960s.

The Green Mountains are hardly unique in suffering the one-two punch of acid rain and climate change, the team says; the same thing is probably happening elsewhere. They also warn that air pollution and tree disease could similarly accelerate the effect of climate change in driving trees up the mountains.

The research was detailed in the Proceedings of the National Academy of Sciences.